An optical head comprising a semiconductor laser is used as a light source in an optical information recording and reproducing apparatus which records information on and reproduces information from an optical disc. In this case, light from the semiconductor laser is applied to the optical disc. However, when the light from the semiconductor laser is applied to the optical disc, the light is reflected toward the semiconductor laser. Therefore, when the semiconductor laser is driven by a dc current and oscillates in a single longitudinal mode, the quantity of light directed toward the optical disc is a sensitive function of the light reflected from the optical disc as shown in Japanese Patent Laid-Open Publication No. Sho. 59-9086. As a result, the signal which is obtained by reproducing the recorded information includes noise from the light reflected from the optical disc. In order to reduce such noise generated by the laser light returning to the semiconductor laser, a high frequency current is superimposed on the dc current which drives the semiconductor laser.
FIG. 3 is a block diagram showing a semiconductor laser driver circuit producing a driving current in which a high frequency current is superimposed on a dc current. An output of a high frequency power supply 10 is applied to the anode of a semiconductor laser 1 through a coupling capacitor C and the cathode of the semiconductor laser 1 is grounded through a resistor R. Light emitted from the semiconductor laser 1 is applied to an optical disc 4, through which information is recorded or reproduced, through a collimator lens 2 and a condenser lens 3. In addition, the light emitted from the semiconductor laser 1 is received and converted to a signal by a photo detector 5. The signal is input to a current-voltage conversion amplifier 6 and the output thereof is input to a buffer amplifier, 7. The output of the buffer amplifier 7 is connected to the cathode of a diode 8a of a minimum signal detecting circuit 8 and the anode of the diode 8a is connected to an input terminal 9a of a differential amplifier 9. A reference voltage +V.sub.B is applied to the anode of the diode 8a through a resistor 8b and a capacitor 8c connected in parallel. A reference voltage +V.sub.R is applied to another input terminal 9b of the differential amplifier 9. An output terminal 9c of the differential amplifier 9 is connected to the anode of a diode 11a of a clamping circuit 11 and the cathode of the diode 11a is connected to a driver circuit 12 having a dc power supply therein. In addition, an output of the pulse generator 13 is applied to the cathode of the diode 11a through a a resistor 11b and a capacitor 11c connected in parallel the clamping circuit 11. The output of a driver circuit 12 is applied to the anode of the semiconductor laser 1 through a low-pass filter F.
The operation of the semiconductor laser driver circuit will be described in detail hereinafter.
When a dc current is applied from the driver circuit 12 through the filter F and a high frequency current is applied from the high frequency power supply 10 through the capacitor C to the semiconductor laser 1, the high frequency current is superimposed on the dc current, whereby the semiconductor laser 1 is driven and emits light The light from the semiconductor laser 1 is applied to the optical disc 4 through the collimator lens 2 and the condenser lens 3, thereby recording information on or reproducing information from the optical disk.
FIG. 4 is a graph showing differential quantization efficiency .eta. by showing the relation between driving current I and laser output P of the semiconductor laser 1. More specifically, the differential quantization efficiency .eta. of the semiconductor laser A is different from that of the semiconductor laser B. In addition, the differential quantization efficiency .eta. varies with ambient temperature. For example, when the ambient temperature is increased, the slope of the line showing the differential quantization efficiency .eta. of the semiconductor laser A is decreased as shown by a dashed line X. Therefore, when the same driving current flows, the output level of the laser light is reduced from L.sub.1 to L.sub.2.
A description will be given of dc current driving level setting means for automatically setting a power level of the semiconductor laser. FIG. 6 shows signal waveforms at each node n.sub.1 to n.sub.6 of the dc current driving level setting means. The pulse generator 13 receives a sector detecting signal S.sub.D which is obtained by detecting the start of each sector (not shown) of the optical disc for recording and reproducing information and a test pulse T.sub.P is generated at the head position of each sector using the sector detecting signal S.sub.P as a reference. In addition, a gate signal S.sub.G is applied from the pulse generator 13 to the gate circuit 14 so that while the test pulse T.sub.P is applied, the high frequency current is not applied to the semiconductor laser 1. On the other hand, the square-wave test pulse T.sub.P for setting the power level of the semiconductor laser 1 is applied from the pulse generator 13 to the clamping circuit 11 and then applied to the driver circuit 12 through the parallel circuit of the resistor 11b and the capacitor 11c of the clamping circuit 11.
The light emitted from the semiconductor laser 1 driven by the driver circuit 12 is received by the photodetector 5 and is converted to a signal that is input to the current-voltage conversion amplifier 6. The currentvoltage conversion amplifier 6 amplifies the voltage converted from the current detected by the photo detector 5 and its output is further amplified by the buffer amplifier 7. Then, it (the signal having an upper peak, i.e., maximum, value V.sub.2 and a bottom peak, i.e., maximum, value V.sub.1 as shown in FIG. 6) is input to the minimum detecting circuit 8. A capacitor 8c holds the voltage difference V.sub.B - V.sub.1 and the minimum detecting circuit 8 detects the bottom peak value V.sub.1 of an output voltage of the buffer amplifier 7 in response to the squarewave test pulse T.sub.P. The detected peak value signal is input to the input terminal 9a of the differential amplifier 9. The differential amplifier 9 compares the bottom peak value voltage with the reference voltage V.sub.R and then amplifies the difference between them and the amplified output V.sub.C shown in FIG. 6 is input to the clamping circuit 11. The capacitor 11c is charged by the difference voltage between the minimum voltage of the test pulse T.sub.P having an upper peak value V.sub.II and a bottom peak value V.sub.I input from the pulse generator 13 and the output voltage of the differential amplifier 9 in the clamping circuit 11. Then, the bottom of the test pulse T.sub.P is clamped to a voltage at which the laser power predetermined by the reference voltage V.sub.R of the differential amplifier 9 is obtained as shown in FIG. 6. Thus the power of the semiconductor laser 1 is automatically set at a predetermined level.
As described above, the semiconductor laser is driven by a current in which a high frequency current is superimposed on a dc current and oscillates in multiplied longitudinal modes, so that generation of noise caused by the light reflected from the optical disc can be prevented.
Meanwhile, as described above, the differential quantization efficiency of the semiconductor laser 1 varies with each semiconductor laser and the ambient temperature.
When the semiconductor laser A is driven, the dc current is set at I.sub.1 and a driving current I comprising a high frequency current I.sub.10 superimposed on the dc current I.sub.1 is applied. When the semiconductor laser B is driven, the dc current is set at I.sub.2 which is larger than the dc current I.sub.1 and the driving current I comprising a high frequency current I.sub.20 which is larger than the high frequency current I.sub.1O superimposed on the dc current I.sub.2 is applied. As a result, the same laser power output level is obtained from both semiconductor lasers A and B.
FIG. 5 is a graph showing the relation between the level of noise generated by a change of light quantity reflected from the optical disc toward the semiconductor laser and a high frequency current superimposed on a dc current. As can be seen from FIG. 5, when the high frequency current I.sub.1O (or I.sub.20) reaches a value so that the difference between the dc current I.sub.1 (or I.sub.2) and a driving current I.sub.11 (or I.sub.12) of the (or I.sub.2) and a driving current I.sub.11 (or I.sub.12) of the semiconductor laser A (or B) when the laser output power P is zero, the noise level NL is suddenly decreased. Even if the high frequency current I.sub.10 is further increased, the noise level NL is not changed. However, if the high frequency current is increased beyond necessity unnecessary electromagnetic wave energy is radiated, adversely affecting the peripheral circuits. On the other hand, if the high frequency current I.sub.10 is decreased beyond necessity, the effect to be achieved by superimposing the high frequency current on the dc current is not obtained.
As described above, when the high frequency current is set at a difference value between the dc current I.sub.1 (or I.sub.2) and the driving current I.sub.11 (or I.sub.12) when the laser output P is zero, the level of noise caused by reflected light can be minimized and also the semiconductor laser can be driven with maximum efficiency. However, since differential quantization efficiency varies with each semiconductor laser and ambient temperature, the high frequency current can not be always applied to the semiconductor laser appropriately.